Television transmitting system



Feb 9, 1943. E. l. ANDERSON ET AL TELEVISION TRANSMITTING SYSTEM Filed Feb. 11, 1941 2 Sheets-Sheet l INVENTORS ZZIIZMIW dwz mdflllemflam ATTORNEY Feb. 9, 1943. E. l. ANDERSON ET AL TELEVISION TRANSMITTING SYSTEM Filed Feb. 11, 1941 2 Sheets-Sheet 2 Ag. m

A TO NEY the light responsive electrode of such tube.

Patented Feb. 9, i943 TELEWSHON TRANSMTTKNG SYSTEM Delaware Application February 11, 1941, Serial No. 378,348

9 Claim.

This invention relates to an improvement in television transmitters and more particularly in transmitters for use in the transmission of color television images.

One method of operation of a color television transmitting system is the transmission, in sequence, of separate images representative of difierent color components of the image, each sequential transmission corresponding to a television field. This is accomplished by projecting an optical image of the object to be transmitted through difierently colored filters upon the mosaic oi the television transmitter tube or upon The scanning operation in the transmitter tube then produces a series of picture signals which .vary in signal strength in accordance with the intensity of that particular color in the object. At the receiver a similar color filter screenais used which is interposed in the optical axis of the projection tube or which is placed in front of the viewing screen of the usual Kinescope. The filters at the receiver are moved in synchronism with the movement of the filters at. the transmitter so that the black and white images which are produced at the receiver may be given proper color tone in order to build up the desired balance of color in the composite color image.

In a television transmitting tube of the Icono-, scope type a charge image is produced on the mosaic electrode in proportion to the intensity of the light projected thereon. When this electrode is scanned by a focused beam of electrons the charge image which exists on the mosaic electrode is used in cooperation with the scanning beam to produce the series of picture sigrials. If each and every element of the mosaic electrode were scanned by the cathode ray beam during each scanning field then substantially all of the charged image would be removed by the beam and the mosaic would be brought to a uniform datum potential level. However, in present television transmitting systems each element of the mosaic is not scanned during a single scanning operation since interlaced scanning is at present used. Accordingly, those elements which are present between the scanned lines on the mosaic for each scanning field do not have their charges removed and consequently there is a considerable retention of the charge image on the unscanned portions of the mosaic after a single field has been scanned.

The storage properties of the Iconoscope are well known and consist of the ability of the tronic scanning beam. If the mosaic is scanned several times following thempplication of a light charge or following the production of a charge image on the mosaic a signal will be produced by each successive scanning operation but the amplitude of the picture signals so produced will decrease with each successive scanning cycle.

In a color television system using sequential scanning and wherein the transmission of the various color components of the object to be transmitted are scanned in sequence, it is essential that the scanning beam remove all charges on the first scannin operation or that someequivalent means or method be exercised to produce an equivalent eflect.- This is necessary since otherwise the picture signals derived from the television transmitting tube would not consist solely of impulses or signals due to that particular color but would be a composite series of signals due to the particular color involved plus that of the previously projected color image since a certain residual charge, as explained above, will be retained by the mosaic electrode.

The efiect of projecting an optical image on the mosaic electrode of a television transmitting tubeis that of releasing photo electrons from th mosaic and it is because of this effect that the charge image is produced.

When the mosaic electrode is scanned by the beam of electrons, the photo electrons which were previously emitted from the mosaic electrode are replaced by electrons from the beam in order to bring the individual elements of the mosaic which are scanned by the beam to a substantially uniform datum potential level. As stated above, in present television transmitting systems each element of the mosaic is not scanned by the cathode ray beam during a single traversal and as a result not all or the photo electrons which were emitted from the mosaicare replaced by electrons from the beam. If such were the case then obviously no residual charge would remain and the next succeeding scanning operation would result in the production of picture signals indicative otthe color content of the particular color involved in that scanning operation., Since the charge image on the mosaic is not entirely removed and since the photo electrons which were emitted from the mosaic are not entirely replaced by electrons from the scanning beam, then in order to bring about the desired result and 'in order to produce proper signals for each sequential color scanning operation it is necessary to provide some means for replacing all of the lost photo electrons after each field scanning operation.

One purpose of the present invention, therefore, resides in the provision of means for replacing all emitted photo electrons of the individual elements of the mosaic and to supply electrons to the individual elements of the mosaic which are deficient in electrons in order to assure a condition of uniform charge on each of the elements of the mosaic following each scanning cycle and preceding the commencement of the next succeeding scanning cycle.

Another purpose of the present invention resides in the provision of one or more sources of electrons in addition to the electron gun structureof the television transmitting tube for supplying electrons to the mosaic electrode to replace the lost photo electrons after each scanning cycle and before the next succeeding scanning cycle.

A further purpose of the present invention resides in the provision of one or more sources of electrons together with .means whereby the sources are ineffective to supply electrons to the mosaic electrode during the scanning operation but which are permitted to supply electrons to the mosaic during a portion of the time interval between successive scanning operations.

Still another purpose of the present invention resides in the provision of means whereby all elements'of the mosaic electrode may be brought to a substantial uniform datum potential level between each successive scanning operation in the transmitting tube.

A still further purpose of the present invention resides in the provision of means whereby the charge image present on the mosaic electrode at any particular time will be properly indicative of the color content of the object for the particular color in question at the particular instant.

Still other advantages and purposes of the present invention will become apparent to those skilled in the art from the following specification, particularly when considered in connection with the drawings wherein:

Figure 1 shows schematically a television transmitter incorporating one form of the present invention.

Figures 2a and 2b show details of difierent forms of color discs to be used in connection with the television transmitter.

Figures 3 and 4 show in detail two forms of the application of the present invention to a television transmitting tube, and

Figures 5, 6, and 7 show potential curves which exist at various points in the television transmitting system.

In Figure 1 of the drawings is shown a television transmitting system which includes a television transmitting tube lil. An electron gun structure is provided in the tube which includes a cathode it, a control electrode M, a first accelerating anode It, and a second accelerating anode I 8. When proper potentials are applied to these electrodes, a focused beam of electrons -Qdirections, electro-magnetic deflecting coils 22 and 2d are provided. These coils, whenjene'rgized with the proper potential variations,

cause deflection of the beam of electrons in horizontal and vertical directions respectively. In order to cause the beam of electrons to be deflected at the desired rates, a master oscillator 26 is included in the system and through the use of frequency dividers 28 and 30 sub-multiple frequencies of the frequency generated by the master oscillator may be produced. The frequency supplied. by the frequency divider 28 may, for example, correspond to thedesired rate of horizontal deflection, that is, the line scanning frequency, whereas the frequency supplied by the frequency divider 30 may correspond to the rate of vertical deflections of the cathode ray beam; that is, the television field frequency. The output from the frequency divider 28 is supplied to an impulse generator 32 for producing impulses of line frequency but of the desired intensity and duration for operating a horizontal deflection generator Si l. The horizontal deflection generator supplies energy to the horizontal or line deflection coils 22. Likewise, the output from the frequency divider 30 is supplied to an impulse generator 36 which produces the desired voltages and impulse duration for controlling the operation of the vertical deflection generator 38.

The output from the vertical deflection generator 3B is supplied to the vertical deflecting coils 2t.

When a light image is projected upon the mosaic electrode 20 and when the electrode is scanned by the produced beam of electrons, picture signals are developed at the signal plate associated with the mosaic electrode and these picture signals are applied to the load resistor ii). The signal plate of the mosaic electrode 2!] also supplies energy to the television amplifier, modulator and transmitter shown as a single schematic element 12. The transmitter, of course, is supplied with impulses from the horizontal and vertical impulse generators 32 and -36 respectively in order.that proper synchronizing impulses as well as picture signals may be used to modulate the television carrier so that the receivers may respond to the synchronizing signals and may be maintained in proper synchronous operation with the television transmitter.

In some instances it may be undesirable to develop picture signals during the return deflection of the cathode ray beam in horizontal and vertical directions and, accordingly, impulses of a predetermined intensity are supplied from the impulse generators 32 and 36 through coupling condensers wand 46 to the control electrode I 4 of the electron gun structure. When impulses of proper polarity and intensity are supplied to the control electrode through these condensers, the cathode ray beam may be substantially entirely extinguished during the return deflection in both horizontal and vertical directions. It is to be understood that the return deflection of the beam in either direction is of the order of seven to ten percent of the time occupied for a complete cycle of operation of deflection of the beam in that particular direction.

In the television system as so far described television images may be transmitted and received in the conventional black and white man ner provided, of course, that optical images are projected upon the mosaic electrode. Since the present television images where the separate color components are to be transmitted in sequence, it is necessary that a color disc or filter device he provided at the television transmitter. Ac-

cordingly, a colored disc 68 is shown and the disc colors, for example red, green and blue.

- is positioned along the optical axis of the television transmitting' tube. The disc is rotated by means of a motor 56 which may be driven directly from the frequency divider 30 for supplying a frequency corresponding to the frame frequency. If the energy derived from the frequency divider is not sufiicient to drive the motor, then the motor may be driven from a separate source of energy but as may well be appreciated it is necessary that the speed of the motor be maintained synchronous with respect to the vertical deflection rate of the cathode ray beam in the television transmitting tube. The phase position of the color disc 68 and its speed of operation with respect of the vertical deflection of the cathode ray beam is such that an optical image is projected on the mosaic through one section of the color disc and this optical projection is immediately followed by a vertical scanning operation which is concluded before the next optical image is projected on the mosaic through the next; succeeding filter section of the color disc.

Figure 2a shows one form of the color disc 48 having'sectors provided with filters of different The number of sectors provided on the color disc may be altered depending upon the desired speed of rotation of the disc and the size of the disc. As shown in Figure 2a, the various filter sectors of the disc are positioned adjacent each other, how ever, if the color disc is located near the lens systern 52 of the television transmitting tube, or preferably between the various lens elements of the lens system 52, the change in the color of the projected image On the mosaic electrode will occur substantially instantaneously. When a color disc of the type shown in Figure 2a. is used, of course, an optical image of one color oranother is continuously projected on the mosaic electrode.

In the present invention a color disc of the type shown in Figure 2b may be used, however, and in this figure the filter segments are reduced in size and are not positioned adjacent each other but are separated by opaque portions 5% of the color disc t8. When sucha disc is used and located near the lens system 52 or between lens elements of the lens system, light will not be projected on the mosaic continuously but separate light images will be intermittently projected thereon in succession, each image having a different color depending upon the color of the filter which is interposed in the optical axis. When such a color disc is used an optical image of a particular color is flashed upon the mosaic electrode, preferably during the vertical return portion of the vertical deflection cycle, and the mo saic electrode is scanned in substantial darkness during the time that the opaque portion 54 of the color disc $81 is interposed in the optical axis.

, Either color disc may be used in the present invention.

' A television system such as described will transmit images in color but the produced images at the television receivers will not have complete fidelity of color and will not be properly balanced in so far as color ratios are concerned. The reason for the lack of infidelity of color in the reproduced image, as explained above, is because of the fact that the charge imageproduced on the mosaic electrode is not completely eliminated or cancelled by a single scanning operation or cycle of the cathode ray beam. Each scanning operation produces signal impulses not only representative of the particular color image which was last projected on the mosaic electrode but also a certain signal component will be present, representative of the colors of previously projected images on the mosaic.

In order that each individual element of the mosaic electrode 20 may be brought to a sub stantially common datum potential level after each vertical scanning cycle, a source of electrons is provided. This source is indicated by the cathode 56 and for heating the cathode to a point where electrons may be emitted thereby, a heater element 58 is also provided. The heater element may, of course, be energized by any appropriate potential means. Associated with the cathode 56 is a control electrode 60 for controlling the number of electrons which may be emitted from the cathode 56. In Figure 1 only a single source 56 of electrons is shown, whereas in Figure 3 a plurality of sources or cathodes 56 are shown. Furthermore, as indicated in Figure 3, each cathode is. supplied with an individual control electrode 60. Normally, the control electrodes to are provided with a negative potential with respect to the potential of the cathode 56 sufiicient to preclude the emission of any electrons beyond the control electrode.

In a television transmitter it is common practice to ground the second accelerating anode l8 and to operate the cathode l2 of the gun structure at a negative potential with respect to ground. The various elements of the gun structure are supplied with proper potentials through the use of a potentiometer or voltage divider 62 one end of which is grounded and the other end of which is supplied with a negative potential as indicated in Figure 1. When the television transmitting tube is so operated the cathode 56 may be connected to ground and the control electrode 60 normally operated sumciently negative with respect to the cathode to preclude the production of electrons.

As previouslydcscribed, the impulse generator 36 supplies voltage variations of a predetermined length and of a frequency corresponding to the vertical deflection frequency of the cathode ray beam. Figure 5 shows a curve representing the wave form which may be supplied by the impulse generator 36 for operating the vertical deflection generator 38 and for supplying the proper synchronizing impulses to the television transmitter. Furthermore, impulses of substantially the same wave form may be supplied .to' the control electrode I4 of the electron gun structure in order to produce blanking of the cathode ray beam during the vertical return cycle. In this instance. however, a reversal of thepolarity is necessary since .the control electrode M must be driven negatively to produce the desired blanking during the return cycle. InJigure 5 the length of time for one complete field scanning cycle is indicated and in this complete time the voltage of the wave form is positive or extends in a positive direction for only one-tenth or less of the complete field cycle. During the remainder of the field cycle the voltage is negative. If such a wave form were to be applied to the control electrode 60 electrons would be supplied to the mosaic electrode 20 from the cathode 56 during the complete return time of the cathode ray beam in a vertical direction. This would be undesirable, however, since it would leave a very limited time for the mosaic to accumulate a charge image representative of the next color to be transmitted. Accordingly, the impulse generator Q6 also includes means whereby impulses of vertical deflection frequency but ofa very short duration may be produced and such means may be in the form of a diilerentiating circuit such 'as shown and described in Schade Patent No. 2,254,626, issued on Sept. 2, 1941. These positive impulses, as indicated in Figure 6, preferably occupy only about 1 to 2% of the entire vertical deflection cycle. When voltages of the wave form shown in Figure 6 are then applied to the control electrode 60, these impulses being so phased that they coincide with the initial portion of the return deflection interval of the cathode ray beam in a vertical direction, the mosaic electrode 20 will be supplied with electrons from the cathode 56 only during a small portion of the beginning of the vertical return deflection of the cathode ray beam. The number of electrons supplied by the cathode 56 and their velocity (accelerating potential) are such as to replace all lost photo electrons which were not replaced by the previous scanning operation of the cathode ray beam. This replacement of the lost photo electrons of the mosaic naturally completely cancels the residual charge image which may be retained by the mosaic electrode after the mosaic has been scanned once in a vertical direction. The cathode 56 is then prevented from supplying electrons to the mosaic electrode by negatively biasing the control electrode 60 with respect to the cathode. During the'remaining portion of the time allotted for the vertical return deflection of the cathode ray beam, a completely new charge image is built up on the mosaic due to the loss of photo electrons in response to the projection of an optical image thereon of thenext color to be transmitted. This charge is then scanned during the next succeeding vertical scanning operation cycle to produce picture signals corresponding to the intensity of the light of the particular color projected on the mosaic electrode.

Actuallyjas stated above, and as indicated in Figure 3, a plurality of sources of flooding elec trons may be provided, each of which has an individual control electrode. The control electrodes are all connected together and are supplied with a voltage of a wave form similar to that shown in Figure 6. The various sources 56 are so situated as to supply electrons over the entire light responsive surface of the mosaic in order to assure complete cancellation of any residual charge and in order to make certain that all individual elements of the mosaic are brought to the same datum potential level.

It may be seen, therefore, thatwhen a system such as that described above is used, each sequential scanning operation produces picture signals properly indicative of a singlecolor only and, accordingly, the color image produced at the receiver will be a more faithful reproduction and balance of the actual colors of the object transmitted.

Instead of using cathodes b6 and associated control electrodes till it is possible in another form of this invention, as indicated in Figure 4, to use only a plurality of cathodes '56 without using any associated control electrodes. If only the cathodes are used they are normally biased positively with respect to ground in order to normally prevent emission to electrons therefrom since the second accelerating anode It and the mosaic electrode 20 are operated at substantially ground potential as indicated in Figure 1. Eli-- i'ectively, therefore, the cathodes W are normally supplied with a positive potential with respect to it the mosaic electrode and with respect to the second accelerating anode l8.

During the initial portion of the return part of the vertical deflection cycle, if the cathodes "56' have applied thereto a negative potential with respect to ground, that is with respect to the mosaic electrode 20 and the second accelerating anode IB, electrons may be omitted from the cathodes 56' and these electrons are then accelerated toward and impinged upon the mosaic electrode 2|] in order to cancel any residual charge image (positive) which may reside thereon because of the previous projection of a light image thereon and because of the inability of the oathode ray beam to completely cancel the charge for the reasons given above. During this short interval the cathodes 56' supplied sufllcient electrons to replace all of the lost photo electrons which have not been replaced by the immediately preceding scanning operation. A wave form of the voltage applied to the cathodes 56' is shown in Figure 7 which is similar to the wave form shown in Figure 6 except that the polarity is reversed.

Since it is not desired that any secondary electrons be produced as a result of the electrons from the flooded sources 56 or 56 reaching the mosaic, the accelerating potential is maintained relatively low. Ordinarily a difference of potential of the order of 10 volts is sumcient so that in Figure 4 where only cathodes 55 are provided, the negative impulses supplied thereto may be of the order of 10 volts or less, In the modification shown in Figure 3, however, the control electrodes may not actually be supplied with a potential which will drive them positive with respect to ground, however, the impulses extend in a positive direction as indicated in Figure 6 and are suflicient to permit electrons to be emitted by the cathodes 56 in suificient quantity to completely replace all lost photo electrons at the mosaic. Normally the difierence of potential between the elements of the mosaic and the cathodes 56 is suiiicient to afford the necessary electron acceleration.

Although the present invention is described as applied'to a television transmitting system for transmitting color images wherein a color disc is provided, it is to be understood that the present invention may as well be applied to systems where the subject matter to be transmitted is projected onto the mosaic by differently colored lens. Furthermore, the present invention may be used in the transmission of color pictures from a moving picture film where successive frame images of the film are of difierent colors, in which case no specific color disc is necessary at the transmitter,

Furthermore, the present invention described is also applicable to other forms of television transmitting tubes and may of course be used where a residual charge on the mosaic electrode is to be eliminated only after two or even more scanning cycles.

Various other alterations and modifications may be made in the present invention without departing-from the spirit and scope thereof and it is desired that any and all such modifications be considered within the purview of the present invention except as limited by the hereinafter appended claims.

We claim:

1. A television transmitting system comprising a. transmitting tube having an electron gun structure for generating a cathode ray beam and a, light responsive electrode, means for projecting an optical image onto the light responsive electrode to produce thereon a charge image in accordance withthe intensity of the light projected thereon, means for scanning the light responsive electrode by the generated cathode ray beam in substantially bilateral directions to produce picture signals and to thereby remove at least a part of the produced charge image at predetermined portions of the light responsive electrode, an additional source of diffused electrons included in said cathode ray tube, means for normally preventing emission of electrons from said additional source, and means for permitting an appreciable emission of diffused electrons from said source only for an interval during the return deflection ofthe cathode ray beam whereby the charge image on the light responsive electrode may be completely canceled.

2. A television transmitting system for transmitting sequential color images comprising a transmitting tube having an electron gun structure for producing a cathode ray beam and a light responsive electrode, means for projecting an optical image onto the light responsive electrode to produce thereon a positive charge image in accordance with the light values of the image projected thereon, means for deflecting the produced cathode ray in bilateral directions to scan the light responsive electrode to produce picture signals and to thereby cancel predetermined portions of the produced charge image, an additional source of difiused electrons included in said cathode ray tube, means for normally prevent-' ing the emission of electrons from' said additional source, and means for permitting an appreciable emission of a flooding stream of dilfused electrons from said additional source only during a portion of the return deflection interval of the cathode ray beam in one direction of deflection whereby the positive charge image on the light responsive electrode may be completely obliterated.

3. A television transmitting system for transmitting sequential color images comprising a transmitting tube having an electron gun structure adapted to generate a focused cathode ray beam andv a light responsive mosaic electrode, means for projecting an optical image on the light responsive mosaic electrode to produce thereon a positive charge image in accordance with the intensity of the light image projected thereon, means for cyclically scanning the light responsive mosaic electrode by the generated cathode ray beam to produce picture signals and to concurrently remove a part of the produced the residual positive charge image on the lightv responsive electrode may be completely obliterated after each scanning cycle.

4. A television transmitter including a transmitting tube having a gun structure for generating a focused beam of electrons, a light responsive electrode in said tube, means for projecting an optical image upon said light responsive electrode to produce thereon a charge image representative of the optical image, means for deflecting the generated cathode ray beam in substantially horizontal and vertical directions to scan said light responsive electrode in order to produce a series of picture signals, the scanning operation also being effective to remove a part of the produced charge image, an additional source of difiused electrons in said tube for flooding the surface of the light responsive electrode with electrons, means for normally preventing emission of electrons from said additional electron source, and means for permitting a relatively intense flooding stream of electrons to be emitted from said additional electron source for a portion of the vertical return time of the cathode ray beam to remove the remainder of the produced charge image.

5. A television transmitter including a transmitting tube having a gun structure for generating a focused beam of electrons,mea fordeflect ing the generated cathode ray beam in substantially horizontal and vertical directions, a light responsive mosaic electrode in said tube, means for projecting an optical image upon said light responsive electrode to produce thereon a positive charge image through the loss of photo electrons,

means for scanning aid light responsive mosaic electrode by said horizontal and vertical deflections in order to produce a series of picture signals, the scanning operation also being effective to replace a portion of the lost photo electrons, an additional source of electrons in said tube positioned near the light responsive electrode for flooding the surface of the light responsive electrode with electrons, means for normally preventing emission of a relatively intense stream of electron from said additional source, and means for permitting the relatively intense stream of electrons to be emitted from said additional electron source for a portion of the vertical return time of the cathode ray beam to replace all previously lost photo electrons that remain unreplaced after the scanning operation.

6. A television transmitter including a transmitting tube Having a gun structure for generat ing a focused beam of electrons, means for deflecting the generated cathode ray beam in substantially horizontal and vertical directions, a light responsive mosaic electrode in said tube, means for projecting an optical image upon said light responsive electrode to produce thereon a positive charge image through the loss of photo electrons, in accordance with the intensity of the light image, means for cyclically scanning said light responsive mosaic electrode by. said horizontal and vertical beam deflections in order to produce a series of picture signals by a removal of a portion of the charge image, an additional source of difiused electrons in said tube positioned near one side of the light responsive mosaic electrode for flooding the surface of the light responsive electrode with a relatively intense stream of electrons, means for normally preventing emission of electrons from said additional source, and means for permitting the relatively intense stream of diffused electrons to be emitted from said additional electron source for a portion of the vertical return time of the oathode ray beam to replace all lost photo electrons and to bring all elements of the light responsive Inosiaic electrode to an equal datum potential eve 7. A television transmitting system including a transmitting tube having a gun structure for generating a focused beam of electrons, a light responsive electrode positioned in said transmitting tube, means for projecting an optical image on said electrode to produce a positive charge image thereon, means including horizontal and vertical beam deflecting means for causing the generated cathode ray beam to scan the electrode to produce a series of picture signals, the scanning operation resulting in a partial cancellation of the charge image, an additional source of diffused electrons in said tube including a cathode, means for normally preventing electronic emission from said cathode, and means for applying potential impulses of short duration to the cathode during a portion of the vertical re turn time of the cathode ray beam in order to permit a relatively intense-stream of difiused electrons to be emitted from the cathode as a flooding beam directed over the entire light responsive electrode whereby the residual charge image existing on the light responsive electrode after the conclusion of the immediately preceding scanning operation maybe completely cancelled.

8. A television transmitting system including a transmitting tube having a gun structure for generating a focused beam of electrons, a light responsive mosaic electrode sitioned in said transmitting tube, means fo projecting an optical light image on said mosaic to produce a positive charge image thereon, means including horizontal and vertical beam deflecting means for causing the generated cathode ray beam to cyclically scan the mosaic electrode to produce a series of picture signals, each scanning cycle resulting in a partial cancellation of the charge image on the mosaic, an additional source of diffused electrons in said tube including a cathode, means including a control electrode associated with said cathode adapted for normally preventing electronic emission from said cathode, and means for applying positive potential 1m- Dulses of short duration to the control electrode during a portion of the vertical return time of the cathode ray beam in order to permit a relatively intense stream of difiused electrons to be emitted from the cathode as a flooding beam directed over the entire mosaic electrode whereby the residual positive charge image existing on the mosaic after the conclusion of the immediately preceding scanning cycle may be completely cancelled and whereby the charge image produced on the light responsive mosaic electrode prior to the next scanning cycle will be properly indicative of the light values of the optical image then projected onto the mosaic electrode,

9. A television transmitting system including a transmitting tube having a gun structure for generating a focused beam of electrons, a light responsive mosaic electrode positioned in said transmitting tube, means for projecting an optical light image on said mosaic to produce a positive charge image thereon, means including horizontal and vertical beam deflecting means for causing the generated cathode ray beam to cyclically scan the mosaic electrode to produce a series of picture signals, the scanning operation resulting in a partial cancellation of the charge image present on the mosaic electrode, an additional source of diffused electrons in said tube including a plurality of cathodes, means including a plurality of control electrodes associated with said cathodes for normally preventing electronic emission from said cathodes, and means for applying positive potential impulses of short duration to the control electrodes during a portion of the vertical return time of the cathode ray beam in order to permit a sufllciently intense stream of diffused electrons to be emitted from the cathodes in the form of a flooding beam directed over the entire mosaic electrode in order that the residual positive charge image existing on the mosaic after the conclusion of the immediately preceding scanning cycle may be completely cancelled and whereby each element of the mosaic electrode may be brought to a uniform datum potential level.

EARL I. ANDERSON. ALLEN BARCO. 

